This invention relates to the art of artificial illumination for image capture, and more particularly to a new and improved system and method for detecting indirect reflecting surfaces in flash illumination.
The use of auxiliary sources of illumination such as electronic flash is well established. In addition, the further refinement of such devices to provide indirect flash is also well known. Indirect flash illumination eliminates many of the undesirable characteristics of direct lighting such as: harsh shadows, specular reflections, red-eye, and high contrast or loss of tonal detail. Unfortunately indirect flash also requires much more energy than direct flash illumination. Therefore, it is desirable to combine flash systems which contain both direct and indirect flash capability in a single unit.
Circuits have been developed which control the total energy output of direct and indirect flashtubes by a photodetector. The ratio of indirect to direct illumination is fixed, e.g. 75:25, but the total energy output is regulated. The photodetector is positioned such that the axial scene luminance is measured. When a preset signal level is reached, the flash output is discontinued or quenched. This type of scene reflectance based quenching system suffers from several deficiencies. First, the accuracy of exposure control is directly related to the reflectance of the subjects being photographed. If a highly reflective object, e.g. a bride in a white gown, is being photographed, much of the flash illumination will return to the flash sensor, and the light output will be quenched, that is less than the total amount of light is delivered to the subject. A less reflective object, e.g. groom in dark suit, positioned the same distance from the camera will return less light to the sensor, and the light output will not be quenched. As a result, two subjects which should have received equal illumination--located equal distances from the flash source--will actually receive different illumination and the resulting exposures will not be matched. In addition, the proximity of the background surface, e.g. a wall and its reflectance, will also influence the quenching accuracy and add variability to the exposure control process.
A potentially more serious limitation with the afore-mentioned approach requires that the user predetermine the presence of a suitable indirect reflecting surface, e.g. ceiling, for the indirect flash source, prior to composing and taking the picture. Inexperienced photographers often are not aware of the scene conditions and prefer to take pictures without making any decisions. These users require an automatic method for activating the indirect flash system when a suitable reflecting surface is available.
A method and apparatus for automatically apportioning light from a dual tube, direct-indirect flash system is disclosed in U.S. Pat. No. 4,384,238 issued May 17, 1983 and assigned to the assignee of the present invention. The indirect tube is fired first, after which the intensity of the axial light returning from the scene is evaluated. If the signal is strong, the presence of a ceiling is assumed and additional energy is applied to the indirect flashtube. If the signal is below a specified threshold, the absence of a ceiling is assumed, and the energy is sent to the direct flashtube. When presented with a scene of average reflectance this device will provide automatic selection of indirect flash without user intervention. Unfortunately, scene reflectance variability not only can produce quenching induced exposure errors, but it also can interfere with the detection of a ceiling. For example, if an object of high reflectance is located near the camera, a strong return from the indirect flash will be measured, and the presence of a ceiling will be assumed. If an object of low reflectance is positioned in the same place, the return from the indirect flash will be of lower intensity, and the ceiling will not be detected. This can cause potential indirect flash opportunities to be missed.
It would, therefore, be highly desirable to provide a system and method providing artificial flash illumination for image capture wherein detection of an indirect reflecting surface is automatic so as to not require user intervention and is not influenced by the reflectance of the subject.